The invention relates to electron optics.
In early types of color television picture tubes, the electron beam generating systems consisted of identical rotationally symmetric electrodes for each electron beam, arranged next to each other. Later on, in the so-called "unitized gun", in the electrode structure of which the individual electrodes are combined, rotationally symmetric electrodes of the same function lying next to each other were replaced by one common electrode for all beams. Such electrodes are asymmetrical, thus representing a different surrounding for the outer and the center beam. It is customary, therefore, to reduce these differences in the grid 3 and the grid 4, which normally form the electrodes of the main focusing lens, by providing one individual ring for each of the three electron beams. The three rings are mounted next to each other in a surrounding electrode part designed as a pot-shaped envelope. Accordingly, rotational symmetry is almost safeguarded inside these rings with respect to the individual electron beams, but the central and outer beams have different surroundings when entering into and emerging from the areas of the rings. On account of this, the shape of the electrostatic focusing field is not only not completely rotationally summetrical, but is not completely rotationally symmetrical to a different extent with respect to each of the central and the outer beams.
It is the object of the invention, with respect to the electrodes forming the electrostatic focusing lens in an electron beam generating system employing combined individual systems, to eliminate differences in the focusing fields for the central and outer beams within the scope of an electron optical improvement of the electrode structure.